1. Field of the Invention
The present invention relates to a sheet feeding device which feeds a sheet from a sheet stack and to an image forming apparatus in which the sheet feeding device is incorporated.
2. Description of the Related Art
A related art image forming apparatus, such as a printer and a copier, is provided with a sheet feeding device which separates stacked sheets one by one and feeds the sheets to an image forming unit. The most common sheet feeding device feeds a sheet in the following manner: a tray carrying a stack of sheets is raised by a pressurizing unit, such as a spring, a sheet is pressed against a paper sheet feed roller, and the paper sheet feed roller is rotated to send the sheet out.
FIG. 8 illustrates an example of a related art sheet feeding device provided with, in a paper cassette 55, a tray 51 which is rotatable about an axis of rotation 56. Sheets S are stacked on the tray 51. The tray 51 is raised by the elastic force of the spring 53 and upper surfaces of the stacked sheets S are pressed against the paper sheet feed roller 54. In this state, the paper sheet feed roller 54 is rotated and the sheets S are sent out sequentially from the topmost sheet.
In order to feed a sheet in a stable manner in such a sheet feeding device, it is necessary to keep paper feeding pressure (i.e., contact pressure between a sheet and a paper sheet feed roller) within an appropriate range as much as possible regardless of sheet size, sheet type (e.g., basic weight) and an amount of stacked sheets. This is because high paper feeding pressure may easily cause multi-page feeding in which two or more sheets are fed at a time and low paper feeding pressure may easily cause defective feeding in which sheets are not successfully fed.
Paper feeding pressure P in the sheet feeding device is expressed by the equation below:P=F−W   (1)where F represents pressure force of the pressurizing unit which urges the tray upward and W represents the initial total weight of the sheets stacked on the tray. In a configuration illustrated in FIG. 8, as the sheets are fed, the amount of stacked sheets (height) on the tray 51 is reduced and, as a result, pressure force F and the initial total weight W of the sheets are also reduced. An amount of change ΔP in paper feeding pressure is expressed by the equation below:ΔP=ΔF−ΔW   (2)where an amount of change in F is represented by ΔF and an amount of change in W is represented by ΔW. If ΔF and ΔW are equal to each other, it is possible to keep paper feeding pressure to a substantially constant value even if the amount of stacked sheets changes. However, paper feeding pressure usually changes as the number of sheets on the tray decreases depending on sheet size and paper type (basic weight) of the sheets stacked on the tray.
For example, suppose that the same number of DIN A3 sheets and DIN A5 sheets of the same paper type (i.e., of the same thickness) are stacked separately on a tray. The height of stacked sheets is the same in both cases; thus, the length of the spring, i.e., pressure force of the pressurizing unit, is equal in both cases. Paper feeding pressure PL and PS are expressed by the equations below:PL=F−WL   (3)PS=F−WS   (4)where PL represents paper feeding pressure at the time that sheets of DIN A3 size are stacked, WL represents the total weight, and PS represents paper feeding pressure at the time that sheets of DIN A5 size are stacked, and WS represents the total weight. As known from the equations (3) and (4), a difference (WL−WS) in the total weight of the sheets produces a difference in paper feeding pressure.
In order to address this problem, a sheet feeding device has been proposed in which pressure force with which a tray is raised is adjusted depending on sheet size and paper type (basic weight), and feeds the sheets. For example, a paper cassette is usually provided with side guide(s) which defines positions of side edges of the sheets stacked thereon. As an alternative, a configuration has been proposed in which pressure force of the pressurizing unit is changed in cooperation with the movement of the side guide which is moved depending on sheet size of the stacked sheets as described in Japanese Patent Laid-Open No. 6-87540.
Japanese Patent Laid-Open No. 2006-56685 proposes a sheet feeding device which changes pressure force by stretching a spring which is a pressurizing unit by a motor depending on sheet size and paper type (basic weight). In this sheet feeding device, pressure force of the pressurizing unit is adjusted by stretching the spring on the basis of an amount of rotation of the motor being selected from a data table stored beforehand in accordance with the input of paper type from a user.
Japanese Patent Laid-Open No. 9-194050 proposes a sheet feeding device in which pressure force is changed by stretching a spring as a pressurizing unit which raises a tray using a motor; in which sheet feeding device, sheet size, paper type and the number of stacked sheets are detected instead of user input. Pressure force of the pressurizing unit is adjusted by stretching the spring by an amount of rotation of the motor corresponding to the detection result of sheet size, paper type and the number of stacked sheets.
However, in the configuration proposed in Japanese Patent Laid-Open No. 6-87540 in which pressure force is changed in cooperation with the movement of the side guide, adjustment can be made regarding only change in sheet size and is not made regarding change in paper type (basic weight) and the number of stacked sheets.
Configurations described in Japanese Patent Laid-Open Nos. 2006-56685 and 9-194050 have the following matters to be addressed. In order to finely adjust pressure force regarding various sheet size, paper type (basic weight), the number of stacked sheets and the like, sheet size, paper type (basic weight) and the number of stacked sheets should be input correctly by a user or should be detected correctly. However, in a currently-used image forming apparatus in which various types of sheets are used, it is significantly difficult to prepare data tables corresponding to each type of the sheets or to automatically detect all the paper types.
That is, a design in which a user inputs the type of sheet to adjust pressure force as described in Japanese Patent Laid-Open No. 2006-56685, it is necessary to correctly input the sheet density and thickness: such an operation is complicated to the user. In addition, determining pressure force correctly and appropriately requires huge amount of data which have been obtained experimentally.
In the configuration described in Japanese Patent Laid-Open No. 9-194050 which automatically detects sheet size, paper type and the number of stacked sheets, a detailed embodiment regarding a method of detecting sheet size and the number of stacked sheets (height) is described; but there is no description about a method of automatically detecting paper type. In other words, correct determination including paper type is not proposed in the configuration described above.